This invention relates to a process for purifying alkyl methacrylates. In one aspect, the invention relates to the anionic polymerization of alkyl methacrylate monomers.
Alkyl methacrylates may be polymerized by either anionic or free radical mechanisms. A problem with free radical polymerization is termination of the polymerization by disproportionation or by combination reactions. These untimely terminations of the polymer growth cause free radically polymerized polymers to have broad molecular weight distributions. These termination mechanisms and other limitations associated with free radical polymerization also prevent preparation of well defined block copolymers and star shaped polymers by free radical polymerization.
Preparation of alkyl methacrylate polymers, random copolymers and block copolymers by anionic polymerization is described in U.S. Pat. No. 4,388,448. Anionic polymerization of alkyl methacrylates affords a synthetic route to block copolymers by sequential addition of monomers. By anionic polymerization, macromolecules can be prepared with predictable molecular weights, controlled stereochemistry and narrow molecular weight distributions.
Anionic polymerization of alkyl methacrylates has not been commercially significant, however, because of two major problems. First, anionic polymerization initiators are inherently reactive with the alkyl methacrylate carbonyl group. This problem has been addressed by the use of bulky initiators, by polymerization at low temperature and by the use of polar solvents.
The second major problem with anionic polymerization of alkyl methacrylates is the inherent presence of chain-terminating protonic contaminates in alkyl methacrylate monomers. Synthesis of alkyl methacrylate monomers involves either esterification or transesterification with the starting alcohol which ultimately defines the ester alkyl. These alcohols may form azeotropes with the alkyl methacrylates and are therefore expensive and difficult to separate from the alkyl methacrylates by distillation. Longer ester alkyl methacrylate monomers are particularly difficult to separate from alcohols because higher distillation temperatures can cause thermal polymerization.
As a consequence of the problems discussed above, most current commercial poly(alkyl methacrylates) are produced utilizing free radical initiators. Free radical polymerization is not sensitive to the presence of the trace amounts of alcohol, but does not realize the advantages of anionic polymerization.
A process to purify alkyl methacrylate monomers has been described by McGrath in R. D. Allen, T. E. Long, S. E. McGrath, Advances in Polymer Synthesis, 347-61 (1985), in which monomer was stirred over a drying agent and then subjected to vacuum distillation. Alcohols and remaining protonic contaminates were then titrated with a trialkyl aluminum until a persistent yellow-green complex was formed, and the monomer was then vacuum distilled from the impurities. The yellow-green end point indicates the presence of metal complexes with carbonyls. The trialkylaluminums may react with one, two or three alcohol groups, but will not be available to form complexes while alcohol groups are free due to the great affinity of the metal alkyl for the alcohol group.
A problem inherent with such a use of aluminum alkyls to purify alkyl methacrylates is the possibility that trace oxygen, especially with the presence of UV light, may produce aluminum peroxides, which act as free radical polymerization initiators for alkyl methacrylates. This problem is magnified by the need to distill the monomer from the impurities, and the excess of trialkyl aluminum used.
Purification of higher alkyl methacrylates is particularly troublesome using McGrath's method. Exposure of the monomers to higher temperatures increases the opportunity for initiation of a highly exothermic free radical polymerization. The higher boiling temperatures of longer chain alkyl methacrylates such as 2-ethylhexyl methacrylate and lauryl methacrylate therefore precludes the use of distillation to purify the monomers in the presence of trialkyl aluminums even under reduced pressures.
T. E. Long has suggested the possibility of conducting the polymerizations of alkyl methacrylates in the presence of the resultant aluminum compounds. T. E. Long, PhD Dissertation, Virginia Polytechnic Institute and State University, 211 (October, 1987). This suggestion is not commercially acceptable due to the complexed trialkylaluminum present which is indicated by the yellow-green color present after addition of trialkyl aluminums according to Long's process.
Long also discloses a second monomer purification technique which avoids the need to distill the monomer from the purification by-products. This method incorporates treating dried monomer with triethyl aluminum, then passing the mixture over a column of alumina which has been prewetted with dry tetrahydrofuran. The alumina absorbs the excess trialkylaluminum and reaction products of alcohol and water and triethyl aluminum. The monomer is then degassed several times. Although this method avoids the need to distill the dried monomer, care must be taken to avoid exotherms as the monomer is passed over the alumina. The column purified monomer anionically polymerized to a homopolymer with a molecular weight distribution (Mw/Mn) as determined by gel permeation chromatography (GPC) of 1.59. This was an improvement over Long's control example, where polymers were produced from alkyl methacrylates which were purified only by drying with calcium hydrides (2.04) but still falls short of Long's polymers produced from distillation purified monomers (1.35), and remains unsatisfactory for commercial use. An improved process for purifying alkyl methacrylate monomers is therefore still required.
It is therefore an object of the present invention to provide a process to purify alkyl methacrylate monomers for subsequent anionic polymerization which does not require distillation of the monomer in the presence of potential free radical polymerization initiators. It is a further object of the present invention to provide a process to purify alkyl methacrylate monomers to a purified monomer capable of being anionically polymerized to a polymer with a narrow molecular weight distribution.